1. Field of the Invention
The present invention relates to a lighting device and a vehicle headlamp in which the number of light sources to be lit, from among a plurality of light sources connected in series, can be switched.
2. Description of the Related Art
In recent years, solid state light sources such as light emitting diodes (LEDs) have increased rapidly in popularity such that, for example, LEDs may even be used as vehicle headlamps in place of incandescent lamps such as halogen lamps. An LED is lit by applying a voltage exceeding a forward voltage (a barrier voltage) thereto, but since an LED has a similar load characteristic to a constant voltage load, a forward current may increase continuously when a source impedance is low, and as a result, the LED may break. As a simple solution to this problem, current limitation is performed using a current limiting resistor connected in series to the LED, but in a device requiring a comparatively large luminous flux, such as a vehicle headlamp, a current flowing to the LED is also comparatively large, and therefore a lighting device that performs constant current control using a power conversion circuit is employed.
Further, a lighting device employed in applications such as a vehicle headlamp is typically configured such that the number of light sources to be lit, from among a plurality of light sources, can be switched, thereby enabling switching between at least a driving headlamp (a high beam) and a passing headlamp (a low beam).
A device that uses a plurality of light sources connected in series and includes an active element (a switch) connected in parallel to one light source and in series to another light source is known as this type of lighting device (see Japanese Patent Application Publication No. 2004-136719 (to be referred to hereafter as “Document 1”), for example). In the lighting device described in Document 1, when one light source is not selected, or in other words only the other light source is lit, the active element is switched ON (energized) such that respective ends of the first light source are short-circuited. With the configuration described in Document 1, a part of a plurality of light sources can be lit and extinguished without providing each light source with an individual power supply circuit (a switching regulator).
With the configuration described in Document 1, incidentally, when the active element is switched ON in order to extinguish a part of the light sources, the number of light sources connected in series between output ends of a power supply circuit decreases, leading to a reduction in a required load voltage. However, a time delay occurs in the power supply circuit from a point at which the active element is switched ON to a point at which an output voltage decreases due to an effect of a capacitor provided at an output stage, for example, and as a result, an excessive current may flow to the lit light source immediately after the active element is switched ON. This excessive current may cause a solid state light source such as an LED to deteriorate or malfunction. Further, rapid load variation may cause the power supply circuit to generate unstable output such as ringing.
In response to these problems, a lighting device described in Japanese Patent Application Publication No. 2008-126958 (to be referred to hereafter as “Document 2”) is configured such that a control signal of an active element (a FET) is increased gradually in accordance with a time constant of an integration circuit, thereby gradually increasing an energizing current of the active element until a light source is finally short-circuited. Further, in a lighting device described in Japanese Patent Application Publication No. 2012-28184 (to be referred to hereafter as “Document 3”), an operation to switch an active element (a switching element) for short-circuiting a light source from an open condition to a short-circuited condition and an operation to switch the active element from the short-circuited condition to the open condition are performed more slowly than an output power response operation by a power supply circuit (a DC/DC converter). With these configurations, an excessive current can be prevented from flowing to the light source when the active element is switched ON (short-circuited).
However, a transient characteristic of an active element when shifting from an OFF condition to an ON condition differs among individual active elements, and may also differ among identical active elements due to element variation and temperature characteristics. It is therefore difficult to obtain a constant operating characteristic when switching the active element ON and OFF with a configuration such as those described in Documents 2 and 3, wherein the control signal of the active element is simply slowed down.
Further, in a condition where the control signal is comparatively distant from a predetermined threshold, a typical active element operates in a region (a saturation region or a cutoff region, for example; to be referred to hereafter as a “dead zone”) where an ON resistance does not vary greatly even when the control signal is varied. When the active element is maintained in the ON condition or the OFF condition, the control signal is normally maintained at a value sufficiently distant from the threshold so that the active element operates in the dead zone.
Hence, with a configuration such as those described in Documents 2 and 3, in which variation of the control signal is slowed, a certain amount of time is required to exit the dead zone, and therefore a delay occurs in a point at which the control signal reaches the threshold such that the ON/OFF condition of the active element begins to switch. The delay that occurs at this time increases steadily as a difference between the threshold and the control signal while the active element is maintained in the ON condition or the OFF condition increases. In a case where variation in the ON resistance is highly sensitive to variation in the control signal in the vicinity of the threshold of the active element (an active region), variation in the control signal must be slowed even further to ensure that the ON resistance of the active element is varied gradually, and as a result, the delay increases.